Volumina Jurassica, 2010, Viii: 103–130 A precise ammonite biostratigraphy through the Kimmeridgian- -Volgian boundary beds in the Gorodischi section (Middle Volga area, Russia), and the base of the Volgian Stage in its type area Mikhail ROGOv 1 Key words: Kimmeridgian/Volgian boundary, Volgian SSSP, ammonite faunal horizons, Boreal-Tethyan correlation. Abstract. A detailed study of the ammonite faunal horizons of the uppermost Kimmeridgian–Lower Volgian of the Middle Volga area (Russia) was undertaken at the most complete and well-known Gorodischi section. This section shows a complete transition from the Kimmeridgian to the Volgian and is a possible SSSP candidate for the Volgian Stage. Sixteen faunal horizons have been established between the top of the Eudoxus Zone and the top of the Lower Volgian. Revised descriptions of existing horizons (especially in the Kimmeridgian) are given and description of new horizons: cf. anglicum, aff. rebholzi, zeissi, ilowaiskii, cf. praecursor, sokolovi, pavida, “Franconites”. Oscillations in the ratios of ammonites with different affinities from horizon to horizon reflect short-term changes in the climate and/or palaeogeography. The presence of ammonites with Submediterranean affinities throughout the studied interval enables correlations to be made with the Tethyan ammonite succession. The evolution of the eudemic Subboreal lineage of Sarmatisphinctes has been analysed, and a succession of species through the complete Autissiodorensis Zone has been recognised. This comprises, in ascending order, S. cf./aff. subborealis, S. subborealis, S. zeissi, S. fallax and S. ilowaiskii. Two of the species described, Sarmatisphinctes zeissi and S. ilowaiskii, are new. GEOLOGICAL SETTING shown that it contains no biostratigraphically significant gaps (Kiselev, Rogov, 2005). The ammonite infrazonal biostrati- The Gorodischi section is one of the most famous and graphy of several additional sections in the Kimmeridgian- best-studied sections in the Russian Jurassic. It is located -Volgian (Ki/Vo) transition beds were also studied. These are high on the right bank of the River Volga, c. 25 km north of mostly located in the Middle Volga area and, farther south, Ul’anovsk (Figs 1, 2). It was chosen as the lectostratotype on the borders of the Peri-Caspian Lowland (Fig. 1). for the Volgian Stage by Gerasimov and Mikhailov (1966). A recent study of the Ki/Vo boundary beds at Gorodi- Following a suggestion by Cope (1996) that secondary schi (Rogov et al., 2006) included the collection of magne- standards should be accepted, Zakharov (2003) proposed the tostratigraphic, geochemical and microfossil data that is cur- Gorodischi section as the Secondary Stratotype Section and rently being analysed. A detailed description of the section Point (SSSP) for the Volgian Stage. Although the Volgian has been published (Rogov, Kiselev, 2007): a log of the sec- succession there is thin and condensed, recent studies have tion together with the ammonite data is given here (Fig. 3). 1 Geological Institute, Russian Academy of Sciences, Pyzhevskii Lane 7, Moscow 119017, Russia; e-mail: [email protected] 104 Mikhail Rogov 60 terrestrial conditions shallow shelf seas ocean basin Kimmeridgian- 50 -Volgian sections of the European Russia Middle-Russian Sea 4 3 2 Moscow Ul’anovsk 40 1 Fig. 1. Palaeogeography of the Middle-Russian Sea 6 during the Kimmeridgian-Volgian transition (modified after Thierry, 2000) Kimmeridgian-Volgian sections of the European rus- 30 sia mentioned in article: 1 – Berd’yanka (51o25’55” n; 55o25’07” E), 2 – Gorodischi (54o34’55” n; 48o24’56” E), 3 – Polevye-Bikishiki (55o05’40” n; 47o29’21” E), 4 – murzicy (55o18’15” n; 46o11’40” E), 5 – isady (56o04’40” n; Neotethys 45o07’13” E), 6 – Elton (49o8’56” n; 46o50’49” E) 0 200 km Fig. 2. Uppermost Kimmeridgian of the Gorodischi section (photo by A. Manikin, 2005) Fig. 3. Ammonite succession through the Kimmeridgian- -Volgian transition of the Gorodischi section A – stratigraphical log (bed numbers after rogov, Kiselev, 2007); B – ammonite ranges; C – ammonite assemblages. Boreal ammonites (1–2): 1 – cardioceratidae (Nannocardioceras, Hoplocardioceras, Euprionoceras); 2 – Dorsoplanitidae (Eosphinctoceras, Subdichotomoceras); 3 – Boreal ochetoceratinae (Suboxydiscites); Subboreal ammonites (4–5): 4 – Virgatitidae (Sarmatisphinctes, Ilowaiskya, “Pseudovirgatites”); 5 – aulacostephanidae (Aulacostephanus); Submediterranean ammonites (6–8): 6 – aspidoceratidae (Aspidoceras, Schaireria, Sutneria); 7 – oppeliidae (Neochetoceras, Taramelliceras, Ochetoceras, Paralingulaticeras (Rogoviceras)); 8 – lithacoceratinae (Discosphinctoides, ?Franconites). ammonites with subtethyan affinities marked by bold; Boreal ammonoids are underlined; n – number of specimens A precise ammonite biostratigraphy through the Kimmeridgian-Volgian boundary beds... 105 A B C FAUNAL HORIZON THICKNESS [m] SUBZONE ZONE LITHOLOGY BED NO. SUBSTAGE a 1/6-7 Pseudovigratites N = 79 spp thic puschi y tatum 1/8a,b c spp sp. os Schaireria 1/9a-c neoburgense sp. N = 224 eudos Franconites Tenuic Ps I. pseudoscythica 1/10 ? N = 43 “Franconites” 1/11 Ilowaiskya pavida Sphinctoceras i 1/12 Fontannesiella Ilowaiskya sokolovi ? I. pavida Paralingulaticeras (Rogoviceras) efimovi N = 55 Danubisphinctes sp. olov spp. ? Eosphinctoceras I. sokolovi Sutneria asema N = 47 1/13a,b Sok N = 251 P. (R.) efimovi “Pseudovirgatites” puschi i LOWER VOLGIAN 1/14 N. steraspis Schaireria neoburgense N = 205 1/15 Klimov “Pseudovirgatites” tenuicostatum N. cf. praecursor 1/16-18 Aulacostephanus Ilowaiskya pseudoscythica Sarmatisphinctes 9/43= 1/19 zio N = 159 ilowaiskii cf. praecursor 9/39-42 cf. franciscanum Ilowaiskya klimovi cf. Fallax Sarmatisphinctes 9/38 Neochetoceras steraspis Ochetoceras N = 108 fallax ilowaiskii 9/36-37 9/34-35 Neochetoceras taimyrensis 9/32-33 Taramelliceras cf. Sarmatisphinctes 9/29-31 zeissi Sarmatisphinctes 9/28 N = 100 in Callomon, Cope, 1971, pl. 10, fig. 7) Neochetoceras rebouletianum 9/25-27 Suboxydiscites Sarmatisphinctes subborealis Sarmatisphinctes fallax sp. 9/23-24 Autissiodorensis subborealis N = 112 circumspinosum 9/22 cf. Sarmatisphinctes Subborealis subborealis 9/19-21 subsidens Nannocardioceras N = 39 cf. Sarmatisphinctes zeissi UPPER KIMMERIDGIAN 9/14-18 (= Physodoceras anglicum Sarmatisphinctes rebholzi cf. N = 36 9/13 sp. aff. 9/12 N = 61 ex Neochetoceras Nannocardioceras volgae 9/10-11 Nannocardioceras volgae Schaireria Sutneria Sutneria aff. rebholzi Nannocardioceras krausei N = 74 2 9/9 N = 48 9/4-6 Nannocardioceras subnudatum N. cf. anglicum 1 gr. Neochetoceras N = 43 9/3 Eudoxus marly clay beige clay phosphorite nodules grey clay dark-grey, bituminous clay ammonite accumulations Ammonites: 1 2 3 4 5 6 7 8 106 Mikhail Rogov INfrAZONAL AmmONITE BIOSTRATIgrAPHY subborealis subzone Zeiss, 2003 emend. rogov, herein The ammonite successions in all the sections studied This subzone is marked by the occurrences in large num- show mixtures of Suboreal, Boreal and Submediterranean bers of the Subboreal ammonite genus Sarmatisphinctes, taxa which enable detailed long-range correlations within possibly derived from Discosphinctoides, throughout the south- the Kimmeridgian, as well as between the Lower Volgian, east part of the Russian Platform and as far as central Poland. the Arctic Volgian and the Tithonian Stage. The ammonite The base of the subzone in the area studied is characterized biostratigraphy of the Lower Volgian has been described in by sudden changes in the ammonite assemblages. The rarity detail (Rogov, 2004a); some additional remarks are included of A. autissiodorensis records in the higher parts of subzone here. In contrast, the ammonite biostratigraphy of the Upper and the absence of aulacostephanids in some areas (e.g. Kimmeridgian has been little studied in the area under con- the Elton section) makes the Subborealis Subzone easier to sideration. The ammonite succession has been summarized recognise in the area investigated. Scherzinger and Mitta in a field guide (Rogov, Kiselev, 2007), but without descrip- (2006) have introduced the Volgae Subzone for the same tions of the biostratigraphical units or fossils. The same bed stratigraphical interval on the grounds that “at the present time numbers are used here as in the field guide. it is unclear if the species ‘Divisosphinctes sublacertosus’ and ‘Discosphinctoides subborealis’ are conspecific” (loc. uPPEr KimmEriDGian cit., p. 233). However, “D. sublacertosus”, as noted by Ilovaisky (in: Ilovaisky, Florensky, 1941), occurs below Eudoxus Zone neumayr, 1873 Aulacostephanus. Moreover, S. subborealis offers additional advantages as an index-species for the following reasons: The uppermost part of the Eudoxus Zone in the Gorodis- – it has a wide stratigraphical range within the subzone; chi section is characterized by an ammonite succession that – it belongs to the lineage that is used as the basis for is close to those of other Subboreal regions such as England subzonal and zonal subdivision of the Upper Kim- and Extra-Carpathian Poland. The highest ammonite fau- meridgian and a major part of the Volgian; nal horizon of this zone is marked by an absence of aula- – as index species it has priority over the Volgae Subzone costephanids and a strong Boreal influence. Ammonites at proposal. this level are represented chiefly by the small-sized Nanno- Both subzones – the Subborealis and Volgae subzones cardioceras cf. anglicum (Salfeld) (Pl. 1: 1) and are assigned to – were erected (cf. Zeiss, 2003; Scherzinger, Mitta,